M.M.
I will start with a twin.
He can have a double bed when he is a pre-teen.
Plus I think he is going to be big and need a bigger bed.
I had a double bed when I was a teen and loved it. Gave me plenty of room to spread out when I hung out in my room. I used to read on my bed, look at magazines, my girlfriends & I would play board games sitting on the bed, we'd be on the phone w/each other and I could lounge on my big bed, I'd do my homework spread out on my bed (sometimes I'd use my desk), I'd watch my little tiny TV while laying on my bed so a double was great as a teen (13).